How can I make MSVC debugger step into Qt library source code again?

Activate the modules list, Debug - Windows - Modules, to see what libraries are loaded and if debug symbols are loaded for those libraries. Note that you need to have the debugger active on your process to be able to do this.

In debug mode, the Qt libraries should be suffixed by d. QtCored4.dll for instance. If you see QtCore4.dll on the list, you have linked to the wrong libraries. If it does say QtCored4.dll but under Symbol Status there is something different than Symbols loaded., then that's the reason. You can manually load the symbols here by right clicking on the dll and selecting Load Symbols. It asks for the Pdb file after clicking that.

You can add more pdb file locations, if they weren't loaded but exist, from Tools - Options - Debugging(tree) - Symbols(tree). If you want the symbols to be automatically loaded.

Should it give some error, that the pdb file doesn't match the binary or something like that, you should re-build your Qt debug version, unless you have more pdb files to try. Use the Visual studio command prompt and go to the Qt directory, run configure and nmake. It takes a while, possibly some hours. There was some switch to only build debug version but i forgot where it goes. Note that the build might fail and after that it's a lot of fun www.google.com searches. The best bet might be to get the source only package, I forgot where to get that and it's not easily found. If you know where to get it, tell me please. I vaguely remember that there might be some other way to build the debug version, but this is the only one i know for sure.